Notes on Java's new Z Garbage Collector

The new experimental garbage collector in Java 11 named ZGC is available only on Linux x86_64. Its main attraction is that GC pause times do not increase with heap size, and it is NUMA-aware.

Here's a level 200 video about it, and a more detailed level 300 video.

See blog entry by [Dominik Inführ] (A First Look into ZGC), and [Sadiq Jaffer & Richard Warburton] (Java's new Z Garbage Collector (ZGC) is very exciting). Also see this thread on the r/java subreddit.

Some preparatory steps are required if you want to test this in a big way:

Since it is supposed to benefit from Linux's huge (2 MB) memory pages, create them with:

echo 67584 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

Then verify that they have been created (1 GB is 512 pages, so 67,584 pages is 132 GB)

cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

Raise the system limit on number of memory mappings per process. Edit /etc/sysctl.conf and add the following line (Heap in MB x 1.8):

vm.max_map_count=245000

Reload the config

sysctl -p

Verify that the change took

cat /proc/sys/vm/max_map_count

Increase the maximum number of processes available to a single user:

ulimit -u unlimited

Increase the maximum number of open file descriptors:

ulimit -n 1000000

Since this is an experimental feature, you have to enable it with the -XX:+UnlockExperimentalVMOptions argument. All said and done, here is the command I ended up using:

 nohup /mnt/ephemeral0/jdk-11.0.1/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -Xmx128G -Xms128G -XX:ParallelGCThreads=12 -XX:ConcGCThreads=12 -XX:+AlwaysPreTouch -XX:+UseLargePages -verbosegc -Xlog:gc* -Xlog:gc:gc.log -jar ProcessRequestLog.jar &

I performed a couple of experiments on an AWS z1d.6xlarge instance using RHEL 7.6, OpenJDK 11 and a multi-threaded AEM request.log parser I had written. As you can see from this AWS CloudWatch chart, the ZGC did not perform significantly better than G1GC, at least for the workload I used. Note that the first signature represents the test with ZGC. The AEM request.log had about 500,000 lines in it.

You should see messages such as below at the start: